Fuel cells produce electrical power from a fuel, such as hydrogen, and an oxidizer such as airborne oxygen. Fuel cells may be used to propel vehicles by using the electrical energy to power one or more electrical motors to rotate the vehicle wheels. A fuel cell vehicle produces less pollution and less carbon dioxide than vehicles powered by internal combustion engines, particularly if hydrogen is used as the fuel. Fuel cells have advantages over batteries including the ability to refill a fuel tank in less time than would be required to recharge batteries.
FIG. 1 schematically illustrates a cooling system for a vehicular fuel cell. Fuel cell 10 includes alternating anode layers and cathode layers 12 and 14. When provided with fuel and oxidizer, these layers generate a high voltage between a negative terminal 16 and positive terminal 18. One or more inverters (not shown) convert the direct current (DC) voltage difference between terminals 16 and 18 into a three phase alternating current (AC) voltage between the terminals of one or more electrical motors (not shown) to propel the vehicle. Heat is produced as a by-product of generation of electrical power. To dissipate this heat, cooling fluid is circulated around the anode and cathode layers 12 and 14 and through a radiator 20. Electrical motor 22 drives a pressure pump 24 to force the cooling fluid through fuel cell 10, coolant line 26, radiator 20, and coolant line 28.
Radiator 20 is grounded to vehicle structure. The high voltage electrical system is intended to float with respect to the vehicle structure. In other words, the system is designed to electrically isolate terminals 16 and 18 from the vehicle structure. Toward that end, coolant lines 26 and 28 are fabricated from non-conductive materials. However, unless the coolant liquid is also non-conductive, the coolant itself provides a potential electrical connection. Water has desirable properties as a coolant and the electrical conductivity can be made relatively low, but not zero, by de-ionizing. Known non-conductive fluids have less desirable properties as coolants. The impedance of a conductive path is proportional to the length of the path and inversely proportional to the cross sectional area. In order to increase the impedance of the coolant paths between fuel cell 10 and radiator 20, coolant lines 26 and 28 are long and have a small diameter. Packaging these long lines requires considerable space in the vehicle. Also, the pressure required to force fluid through the circuit also increases with line length and decreases with cross sectional area. Therefore, the dimensions that improve electrical isolation increase the pump pressure required to achieve a desired flow rate.